1. Field of the Invention
The present invention relates to an image forming apparatus having a function for adjusting the position at which an image is formed, and to a method of controlling this apparatus.
2. Description of the Related Art
In general, it is desired that an image forming apparatus form an image at a desired position on a sheet of printing material. With a color image forming apparatus capable of forming an image of a plurality of colors, the color image is formed by superimposing images having a plurality of colors. In order to reduce color misregistration, therefore, it is desired that the positions at which the images of the colors are formed be made to coincide.
In order to reduce color misregistration in a conventional image forming apparatus, image formation position and color misregistration are corrected for by detecting a toner pattern that has been formed on a transfer belt using toner. Japanese Patent Laid-Open No. 6-18796 proposes a method of detecting the toner pattern by a CCD line sensor. Further, Japanese Patent Laid-Open No. 6-118735 proposes a method of detecting toner patterns of two or more colors by an optical sensor and then detecting any color misregistration of each color.
According to the examples of the prior art mentioned above, a specular-reflection-type optical sensor is used to detect the amount of light reflected from the substrate (background) of an intermediate transfer member such as a transfer belt and the amount of light reflected from a toner pattern, and the position of the pattern is detected based upon a signal obtained from the difference between the two amounts of light. This means that the difference between the amount of light reflected from the substrate and the amount of light reflected from the toner pattern must be sufficiently large.
FIGS. 17A to 17C are diagrams illustrating the transition of a difference between an amount A of light reflected from the substrate (signal level is high) and an amount C of light reflected from a toner pattern (signal level is low) in a print job involving a large volume of printing. FIG. 17A illustrates the output waveform of the amount of reflected light at the beginning of the large-volume print job, as well as a threshold value for detecting the toner pattern. If the intermediate transfer member has a high gloss, then, at least at the beginning of the large-volume print job, the amount of light reflected from the substrate is obtained and the difference between the amount of reflected light and the threshold value (namely A-B) is acquired to a satisfactory extent. This enables accurate detection of the position of the toner pattern.
However, since the intermediate transfer member becomes progressively contaminated with toner or the like as the number of images formed in the large-volume print job increases, the amount of light reflected from the substrate declines (FIG. 17B). If the contamination progresses, the amount of light reflected from the substrate and the threshold value become equal and erroneous detection of the toner pattern occurs (FIG. 17C). In other words, in accordance with FIG. 17C, a toner pattern exists in the interval in which the amount of reflected light is below the threshold value.
It should be noted that this problem does not readily arise in a case where a small-volume print job, in which the number of images formed is comparatively small, is repeated. The reason is that the intermediate transfer member usually is cleaned at the beginning and end of the print job. However, when several thousand images are formed in a single print job using an intermediate transfer member that is nearly new, the problem described above becomes conspicuous unless cleaning is performed during the printing process. Since executing cleaning results in temporary suspension of image formation, this leads to so-called “downtime” that lowers throughput. It is preferred, therefore, that cleaning not be performed during a print job to the extent possible.